Testing Race Planes

by Jimmy Doolittle

This text was taken from an address delivered in 1933 by Jimmy Doolittle to the Institute of Aeronautical Sciences. This text details Jimmy Doolittle's experiences while testing the Laird Super Solution.

In as much as my work for the past three years has been the developing and selling of petroleum products for aviation usage, I am scarcely in a postion to deliver a truly up-to-date dissertation on scientific flight testing.

With your indulgence, I will however, detail a few personal experiences encountered while testing and grooming a modern racing airplane. Since experimental high speed airplanes differ radically in certain phases of design, admitting numerous specific problems peculiar to each of several types; for the sake of brevity, I shall confine this discussion to the recent Laird 400 ("Super Solution") racer.

In the mid-part of 1931, the ailerons and a good sized piece of right wing fluttered off my Travel Air "Mystery S" airplane which had just been rebuilt, cleaned up and obviously speeded up. This failure was probably due to the Frieze type ailerons being overbalanced at the higher speed or to flexibility in the aileron control tubes.

The failure of my own airplane made available the new racing plane that was built by E.M. Laird and financed by the Cleveland Speed Foundation. this little job had a 21 foot span, 108 square feet of wing area, carried 112-1/2 gallons of gasoline, eight gallons of oil and weighted 1,580 pounds when light and about 2,500 pounds fully loaded. (With the geared engine it weighted about 75 pounds more.)

The first test flight was made from the old Aero Club field, south of the Chicago municipal airport. Laird felt, or hoped, that the high speed of the airplane would be around 300 mph. The geared Pratt and Whitney Wasp Jr., originally mounted in the airplane, developed 560 hp at 2,500 rpm. The propeller was nine feet in diameter and set at 37-1/2 degree pitch at the 42 inch station. Due to the high pitch angle, the ground revs and static thrust were very low.

The airplane ran about a half mile before it could be pulled into the air and then flew for about two miles more before it picked up sufficient speed to come under complete control. In succeeding flights, the propeller setting was reduced 5 degrees and the takeoff was satisfactory though the engine over-revved somewhat (2,600 rpm) in level flight at full throttle.

The tail surfaces were identical with those of the last year's racer. The airplane was stable longitudinally and laterally, but extreamly unstable directionally. This directional instability increased with speed and the airplane was barely manageable at a speed of 200 mph.

This was due to the increased fin area forward, resulting from the longer NACA cowl employed with the geared engine, the large pants and the fairing which filled in the space between the front and rear landing gear struts. All of this fin area was forward of the center of gravity.

The culpability of pants and fairing was proved by removing them and making a flight. The airplane was directionally stable with pants and fairing off. To correct the directional instability, the fin and rudder were increased in height nine inches. Thereafter the airplane was directionally stable, but not as stable laterally as before.

There did not appaear to be any appreciable torque resulting from the large propeller and geared engine except an accelerational torque when the throttle was moved quickly.

Although the pilot was sitting on 50 pounds of lead shot, the airplane was so stable longitudinally that it was dificult to get the tail down in landing and the airplane landed very fast. The fast landing tendencies of this airpalen were attributable largely to the fact that the tail could not be brought all the way down and advantage taken of the maximum angle of attack when landing, and also the blanketing effect of the large propeller disc on the small wings.

This is indicated by the fact that the airplane, when moutning the direct drive engine and eight foot propeller was only slightly lighter, but was less stable, and landed coonnsiderably slower, I should say fully five miles per hour.

In order to ventilate the pilot's cockpit, fresh air was led through a flexible conduit from two holes located in the leading edge of the upper wing. These holes were about three feet from the center line of the wing, located in the slip stream, but well outside the "fume area."

the center of each of these circular holes was somewhat above the center-line of the leading edge and the tops of the holes were aft of the bottoms. In flight the starboard hole blew slightly and the port hole sucked slightly. The efficiency of the ventilating system increased somewhat as the speed increased. The trouble was corrected by putting a scoop on the top of each hole.

The geared engine ran much cooler than the direct-drive. The head temperature averaged from 50 to 75 degrees lower. This appeared to be due to two things. First: the nose of the NACA cowl on the geared engine was longer and tended to direct the air over the cylinder heads better than the blunt nosed NACA on the direct-drive engine. Second: The geared engine was throwing more oil than the direct drive engine. This was indicated by greater oil consumption and higher oil temperature in the use of the geared engine.

At a later date, the direct-drive engine was returned to the factory. The bearing clearances were increased so it would "sling" more oil and overheating difficulties were eliminated. The oil consumption was increased from about one quart to three quarts per hour. This increased oil consumption, however, was not a problem as there was ample oil tankage. Using the oil for a cooling medium caused the oil temperature to run too high and it was necessary to cool the oil by directing cold air taken from in front of the engine over the oil tank.

The fuel used was a straight-run gasoline contraining three cc's of tetraethyl lead and having a knock rating of 87 octane. For full throttle operation, an 89 octane gasoline, containing five cc's of lead was used. This was a safety measure with the 6:1, 10:1 ?Wasp Jr as the 87 octane fuel operated satisfactorialy.

The 87 octane fuel was probably on the border line as I later flew a 6:1, 12:1 Wasp Sr. and with this engien at full throttle operation, the 87 octane fuel started detonating almost at once, and the thermocouple showed a hgead temperature of over 600 deg F. With the 89 octane fuel thre was no detonation and head temperature was stead at about 520 deg F. The 87 octane fuel was satisfactory at cruising speed.

The 400 Racer, was extremely tempermental to rig. It seemed impossible to adjust the landing and flying wires so that it would not be wing heavy on one side or the other. On one flight it balanced perfectly for a while and then gradually became left wing heavy. The maneuver was repeated and the airplane balanced perfectly. Repeated again and actual right wing heaviness resulted. Here was an airplane that could be rigged in flight.

The difficulty was that it wouldn't hold its rig. This fault became continually and rapidly more apparent and annoying. On one flight, in very rough air, the rigging became so flabby that an actual lateral motion of the trailing edge of the upper wing could be observed when the ailerons were moved and when a bump hit one wing more severely than the other.

Before the flight was completed, it was found impossbile to get the airplane out of the left bank at cruising speed without throttling back; so all turns, even around the landing field, were made to the right. In this airplane the main wing truss was incomplete. The auxiliary wing truss had depended upon a fitting around the center of the continuous rear spar in the upper wing to take unevenly distributed wing loads. A careful inspection showed that the spar had crushed at this point and the bolt holes had enlongated.

As a temporary expedient, an eighth inch thick piece of sheet steel was driven between the fitting and the spar to take up the play. This corrected the trouble temporarily, but after a few hours flying it again appeared, due to further crushing of the spar.

The incomplete wing trussing, mentioned above, explains a difdiculty experienced when running the three kilometer speed trials. The airplane was clocked at 255 mph with the direct drive engine. Up to 240 mph, no trouble was experienced, but above this speed a tendency to turn to the left and difficulty in removing the airplane from a left bank was observed. A right bank at full throttle gave very little trouble.

The direct drive engine was removed and the 3.2 geared engine installed. The air-speed indicator showed about eight miles per hour more speed with this engine. There was a very strong tendency to roll to the left. The airplane was then rigged very right wing heavy in order to correct this tendency at full throttle, but it is doubtful if it could be handled in a race.

The course was flown in practice for a few laps at 240 mph, then the throttle was gradually opened until wide. A teammate on the ground was instructed to watch until I rocked my wings, and then to time the next lap, as it would be the only one at full throttle.

Coming down the home stretch, I rocked the stick laterally but the rolling motion of the airplane was so slight that I was afraid my ground observer would not be able to notice it. The 10 mile course in 1931 was an irregular pentagon. The first two pylons were executed successfully but at the thrid, where the angle was sharper, the left wing would not come up and I was unable to recover from the bank until after overturning. Rolled in to a steep right bank to get back on the course and had difficulty getting out of the right.

Banked to the left of No. 4 pylon. The bank increased even with controls reversed and it was necessary to throttle to regain control of the airplane. Obviously the geared engine could not be used in the Thompson trophy race. Not beacuse of any torque difficulties but because the wing warping at high speeds induced the aileron reversal. This condition became critical at about 260 mph.

In order to check the speed of the airplane with geared versus direct-drive-engine, an attempt was made to fly the three kilometer course. On the first run the airplane gradually rolled to the left until out of control. Throttled and tried again. This time entered the course with the right wing down about 30 degrees. After about one kilometer, the airplane was level and at the two kilometer mark the left wing was down some 30 degrees and depressing rapidly.

I throttled and landed, unable to make even one run across the three kilometer speed course. It would have been possible to make runs at say 220 mph and 250 mph, note the corresponding rpm's; plot air-speed versus rpm and, knowing that the maximum level flight rpm was 2600, produce the curve and obtain the high speed.

However the Thompson trophy race was scheduled for the next day and flights previously made indicated that the direct drive engine could surely be handled, though sloppily, on the course. The engines were changed overnight and the morning of race day spent trying to right the airplane.

The difficulty experienced in test flights and in the race itself indicated that something was loosening up and that the wing warping tendencies were rapidly becoming worse. The airplane was finally rigged very right-wing-heavy to facilitate getting out of left banks and entering iin the race. It was forced down in the seventh lap due to overheating and piston failure.

Had we been able to use the cooler running geared engine; had the wing trussing been complete, or the center rear cabane fitting more secure so that the wings couldn't warp; had we known as much then as we know now, none of these difficulties would have arisen; but that is experience.

In the 1932 400 Racer, thse inherent defects were corrected as follows:

1. A longer sharper nose was put on cowling. The engine was adjusted to throw more oil and the oil tank was air cooled.
2. Wing trussing was redesigned and made complete.
3. The pilot was raised ten inches so he could see over the upper wing. A slide door was arranged in cockpit covering so pilot could stick his head and shoulders out and see ahead when landing.
4. To increase gas capacity, the fuselage was fattened out near the center of gravity without decreasing fineness ratio or streamlining.
5. A controllable pitch propeller greatly improved take-off characteristics.
6. The center of gravity was moved aft to correct the excessive longitudinal stability.
7. A retractable landing gear was designed and incorporated to increase speed.
8. The cockpit ventilation intake holes were put exactly on the center line of the leading edge.

It seemed that we had corrected all faults in the original design and the first test flight of the redesigned airplane tended to prove our belief correct until time came to land. Then a new problem arose. The landing gear, in ground tests, dropped all the way out and then was spread and locked into place.

In actual flight, the air loads and rotation of the slip stream spread the gear before it had dropped out, locked it in an intermediate position and it was necessary to make the first landing on the bottom of the fuselage without landing gear. This was corrected by means of a rubber shock cord which held the wheels together until the telescoping struts were fully extended.

In later flights, it was found that the tail fluttered badly when gliding in slowly for a landing. The exact reason for this has not, as yet, been determined, but an effort is now being made to correct it through the use of larger rear fillet between the lower wing and the fuselage. It may or may not work.

there is no work as intensely interesting as testing and improving high speed airplanes. Not even air racing. But I have yet to hear of the first case of anyone engaged in this work dying of old age.